This invention relates generally to a device and method for separating or concentrating different isotopologues of water. Isotopologues are molecules that differ only in their isotopic composition. Hydrogen-related isotopologues of normal or “light” water (H2O) include “semi-heavy water” having a single deuterium isotope (HDO or 1H2HO), “heavy water” with two deuterium isotopes (D2O or 2H2O), tritiated water having a single tritium isotope (HTO or 3HOH) and “super-heavy water” (T2O or 3H2O). For purposes of this disclosure, the term tritiated water will be used to refer to any water molecule in which one or both hydrogen atoms are replaced with a tritium isotope. Tritiated water is a byproduct of nuclear power generating stations. The present invention is especially useful as a method of enriching tritiated water for disposal by evaporation and concentration.
Tritium is chemically represented as T or 3H and is a radioactive isotope of hydrogen. Tritium is most often produced in heavy water-moderated nuclear reactors. This production occurs when deuterium (heavy water) captures a neutron in a reaction having a very small cross section, producing tritium in the form of tritiated water (HTO). Relatively little tritiated water is produced. Nevertheless, cleaning tritiated water from the moderator may be desirable after several years of operation of the nuclear station to reduce the risk of tritiated water escaping to the environment. Very few facilities exist that can properly clean or separate tritiated water from a solution or mixture of tritiated water and normal water. The scarcity of facilities makes it necessary to transport relatively large volumes of contaminated water solution containing relatively small volumes of tritiated water across long distances to a location such as Ontario Power Generation's Tritiated Water Removal Facility. Ontario Power's facility can process up to 2.5 thousand tons (2,500 Mg) of contaminated heavy water per year, producing about 2.5 kg of tritiated water.
Tritiated water is produced in pressurized light water reactors as well. The prevalence is directly related to the use of Boron-10 as a chemical reactivity shim. A shim is used to convert high energy neutrons to thermal heat. The production of this isotope follows this reaction:5B10+0n1->[5B11]*->1H3+2(2He4).
The half-life of tritiated water is 12.4 years. This is troublesome because it is persistent enough to concentrate in the reactor water. Tritiated water causes no ill reactivity effects within the nuclear reactor, but it does provide a significant risk for contamination from small leaks.
Tritium is chemically identical to hydrogen, so it readily bonds with OH as tritiated water (HTO), and can make organic bonds (OBT) easily. The HTO and the OBT are easily ingested by consuming contaminated organic or water-containing foodstuffs. As tritium is not a strong beta emitter, it is not dangerous externally, however, it is a radiation hazard when inhaled, ingested via food or water, or absorbed through the skin. In the form of tritiated water molecules, it can be absorbed through pores in the skin, leading to cell damage and an increased chance of cancer.
HTO has a short biological half life in the human body of 7 to 14 days which both reduces the total effects of single-incident ingestion and precludes long-term bioaccumulation of HTO from the environment. HTO does not accumulate in tissue.
For purposes of this disclosure, the term “water” will be used to refer to H2O, and, where appropriate, other isotopologues will be referred to by name. A solution comprising water and one or more other hydrogen-related isotopologues of water will be referred to as a “contaminated water solution”.
Enrichment of tritiated water by removing the excess water and concentrating the tritiated water can significantly reduce the expense of transporting very low level contaminated materials to a cleaning facility. The available processes are not commercially attractive when starting with low concentrations of tritium as tritiated water because of the transportation costs. No low cost processes have been demonstrated for the concentration of tritiated water due to the fact that it has physical and chemical characteristics that are so similar to water that it precludes normal chemical or thermodynamic measures. These close similarities make it difficult to define processes that will separate the tritiated water from water.
In fact, the preferred sorbents utilized in the sorbent/sorbate working pair adsorption process of the present invention function regardless of whether pure water functions as the sorbate or whether tritiated water or a contaminated water solution function as the sorbate. The adsorption/desorption process at the heart of the present invention will work equally well with all hydrogen-related isotopologues of water.
However, the method and device of the present invention relies on one of the physical characteristic differences between tritiated water and water. The freezing point temperature of tritiated water is +3.8° C., whereas normal water (H2O) freezes at 0° C. under normal atmospheric conditions. The tritiated water will freeze to a solid state when subjected to temperatures less than +3.8° C. Similarly the freezing point of D2O is also +3.8° C. In contrast, the “pure” or “normal” water in contact with the frozen tritiated water (HTO or T2O or the frozen deuterium water (HDO or D2O)) will remain as a liquid until the temperature is further reduced to 0° C.
Attempting to separate the tritiated water from water by freezing the tritiated water and then using mechanical pressure filtration to remove the water would likely prove unsuccessful because of the energy introduced into the water during the pressurization process necessary to force the water through the filters. The increased energy would be sufficient to melt the tritiated water and allow it to pass through the filters with the water.
Co-pending U.S. patent application Ser. No. 12/634,449 entitled “Single Chamber Adsorption Concentrator,” by Avery, et al., discloses a method and system that utilizes low grade heat to drive a sorbent/sorbate working pair to separate a solvent from a solute/solvent mixture (the '449 application). One preferred application of the device described in the '449 application is separating water from the salt brine produced by the aluminum smelting industry. The brine solution is introduced into a single chamber shell proximate the concentrator evaporator where the water in the brine can freely evaporate and the resulting water vapor freely flow without inhibition to be either absorbed into the adsorbent modules or condensed by the condenser. The free flow of water vapor is facilitated by continuous operation of the condenser and by maintaining the brine solution at a higher temperature than the cooling fluid driving the condenser. A mist eliminator with a wash down feature located intermediate to the evaporator and the adsorption chamber is provided to collect contaminants that may be carried aloft from the evaporator by the vigorous boiling and rapid vapor movement.
Co-pending application Ser. No. 12/550,290 entitled “Improved Adsorbent—Adsorbate Desalination Unit And Method,” (the “'290 Application”), describes an open loop adsorption concentrator system having an internally divided housing and utilizing silica gel and water as the preferred working pair. The '290 Application introduces an economizing heat exchanger and a mist eliminator as new techniques to handle the needs of such an open loop system. As with prior art adsorption chillers, the pressure vessel of the '290 Application is a multi-chambered shell interconnected by a plurality of valves which open and close to intermittently prohibit and allow the flow water vapor from chamber to chamber within the pressure vessel. In the '290 Application, the valves are freely actuated by the differential pressure between the various chambers of the concentrator.